The wire effect in a wire-wrapped 37-pin fuel assembly mock-up of a Japanese loop-type sodium-cooled fast reactor (SFR), Monju, has been investigated through a numerical analysis using a general-purpose commercial computational fluid dynamics (CFD) code, CFX. Complicated and vortical flow phenomena in the wire-wrapped 37-pin fuel assembly were captured by a Reynolds-Averaged Navier-Stokes (RANS) flow simulation with a shear stress transport (SST) turbulence model. The CFD results show good agreement with Rehme's friction factor correlation model, which can consider the number of wire-wrapped pins in the fuel assembly. Three-dimensional multi-scale vortex structures start to be formed by an interaction between secondary flows around each wire-wrapped pin. Large-scale and small-scale vortex structures are generated in the corner and edge, and interior sub-channel, respectively. The behavior of the large-scale vortex structures in the corner and edge sub-channel are closely related to the relative position between the hexagonal duct wall and the wire spacer. Regardless of the relative position between the adjacent rod and wire spacer, a small-scale vortex is axially developed in the interior sub-channels. Furthermore, a driving force on each wire spacer surface is closely related to the relative position between the hexagonal duct wall and wire spacer. It is expected that the multi-scale vortex structures in the fuel assembly play a significant role in the convective heat transfer characteristics.
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